The degree of homology of the endogenous human C type proviral sequence compared to C type sequences of Baboon endogenous virus and MuLVs suggested the human provirus represented a new subclass of C type viruses. A probe containing unique human env sequences showed many copies of human env reactive sequences in monkey DNAs. A full-length env reactive provirus was cloned from African green monkey. All regions of this genome sequenced were 80-88% homologous to the human sequences. Analysis of genomic DNA from other old world primates showed all contained human env reactive sequences whereas no new world primate DNA reacted with this probe. Two 100 bp env specific probes subcloned from the N-terminal gp70 region of MCF and xenotropic MuLVs reacted specifically with the respective MuLV. No cross reactivity was detected with ecotropic or amphotropic proviral DNA. Genomic DNA of laboratory mice unexpectedly contained more copies of MCF related sequences than xenotropic MuLV sequences whereas a more diverse pattern was found in wild mouse strains. The development of MCF virus was followed in AKR mice from 5 weeks to 6 months of age by the appearance of MCF related RNA transcripts in the thymus and other tissues. Two mRNA species were expressed exclusively in the thymus (1.8 and 7.2 kb), a 6.0 kb species was expressed in liver and kidney and a 3.0 kb RNA was expressed in all tissues. At three months of age the thymus contained a full-length (8.2 kb) MCF reactive mesage. Two new defective oncogenic viruses were isolated from a splenic tumor in a C58 V-congenic mouse inoculated with C25 LI MCF. One virus studied mapped as an ecotropic MuLV except for a 1000 by ras sequence which replaced late pol and most of gp70. The DNA sequence of the ras showed amino acid 12 was arginine. The p21 coding segment was closely related to bas, H-ras and Rasheed-ras. The putative recombination site for the 5' crossover was identified.